Reverse osmosis: an adaptable solution

Reverse osmosis is a pressurized membrane filtration process, which produces the cleanest effluent of all known wastewater treatment technologies. It is capable of greater than 99.9% removal of biochemical oxygen demand (BOD), chemical oxygen demand (COD), ammonia, metal, inorganic salt and List I and II substances.

Reverse osmosis (RO) exploits the natural phenomenon of osmosis, which is the movement of water through a semi-permeable membrane from a low to a high salt concentration. In an RO plant, this process is reversed by applying pressure against the membrane and reversing the natural movement of water. As water molecules pass through the fine membrane pores, a clean permeate forms on one side of the membrane and contaminants leave a concentrate solution on the other side.

RO technology is most famous for water purification, however, developments in membrane technology has resulted in its widespread use on European landfills for leachate treatment. here are over 70 landfills operating one or more RO plants, either in isolation or in combination with biological treatment plants.

As a physical treatment process, RO has a residence time of just three minutes, whereas for a sequencing batch reactor (SBR) plant it would be 10 to 15 days.

Consequently, an RO plant has a very small footprint. This portability lends the process to much easier volumetric plant upgrades and downgrades, thus making it, I believe, the only commercially available leachate treatment technology that is truly modular and mobile.

Removing the riskThis removes the risk of getting a plant size wrong, an important consideration for landfill operators who won't commit to on-site treatment due to uncertainty about the volume of leachate in a landfill.

A plant with a treatment capacity ranging between 25 to 200m3/day can be delivered in a 40 foot ISO container on the back of a lorry. If required, any plant can be upgraded to a 200 m3/day three-stage plant all within the same container. The price differential for this upgrade is very small compared to an SBR plant. The plant can also be moved to another site, making it a mobile asset.

The absence of extensive civil works means that plant depreciation can be very short compared to the typical 15 to 25 years for a biological treatment plant. Neither is there a large decommissioning cost at the end of the plants useful life. For these reasons, RO plants are suitable for hire, particularly on landfills that may already be tankering leachate off-site at a high cost.

Waste Recycling Group (WRG) has been operating a 200 m3/day Haase RO plant at its Calvert landfill in Buckinghamshire for over 12 months. The plant, licensed to Clarke Energy, is on hire and has replaced leachate tankering. As a self-contained plant housed in a 40-foot container, not fixed to the ground and with only temporary external connections, it was exempt from planning permission.

The plant was delivered, positioned, connected and fully commissioned in less than a week.

The plant has been upgraded in treatment capacity from 100 to 200m3/day. It used to operate five days a week, but now runs continuously seven days a week. Such operation is possible because the resumption of permeate quality only takes a few minutes when the plant is turned back on. Neither has the performance of the plant been affected by its first winter, a common problem experienced by many biological treatment plants.

Flexible for the futureCalvert is a good example of how RO technology can address continually changing treatment requirements, challenging discharge consents, the Environment Agency's requirements for accelerated leachate level reduction and the demonstration of best available techniques (BAT). Currently, concentrate is sent to a sewage treatment works for disposal, however the Environment Agency is working on proposals to return the concentrate back into the landfill, gleaning experience from other successfully operating plants on the continent.

At Calvert this may be advantageous as it is increasingly landfilling drier waste as local authorities reduce the amount of biodegradable waste going into landfill. Concentrate reinfiltration could hydrate the dry waste, accelerate its degradation and assist in the stabilization of the landfill.